The peripheral nervous system extends from the brain and spinal cord to various portions of the body. Two of the main functions of the peripheral nervous system are muscle control and sensory feedback. Peripheral nerves carry signals between the various portions of the body and the central nervous system using electrical signals.
A typical muscle is composed of many thousands of fibers, which contain the contractile machinery of the muscle. Rather than individually controlling each fiber, a single motor neuron can control groups of fibers that form motor units. Motor units vary in size from 100 to several hundred fibers, and also vary in composition of muscle fiber type. Small motor units are typically composed of slow type muscle fibers that are fatigue-resistant, while larger motor units are generally composed of fast type fibers that are easily fatigable and medium sized motor units consist of a mixture of slow and fast fiber types. Motor units are recruited, or turned on, in a specific order that generally begins with the smallest group and progresses to the largest group. In this way, the smaller, fatigue-resistant motor units are used more often, and thus allow for fine force control for longer periods of use. The larger motor units, with larger capacity for generating force, are conserved for brief periods of time when they are most needed, e.g., during a reflex, emergency or other strenuous activities. The size of a motor neuron is correlated to the size of the motor unit that the motor neuron controls, so that a large motor neuron will control a large motor unit.
The normal physiologic recruitment order refers to a typical (healthy) order of motor neuron recruitment, where the size of the motor axons and the motor neuron cell bodies define the sequence of recruitment. For a given synaptic input of current, a smaller motor neuron will be recruited before a larger motor neuron, thus determining the order, small to large.
External electrical stimulation of motor neurons has been attempted. One such attempt stimulates the axon of a motor neuron. This, however, results in a recruitment order that is reversed when compared to the normal physiologic order (the larger motor units are recruited before smaller ones). The implication of this recruitment reversal is that large, fatigable motor units are recruited first, resulting in the loss of fine motor control and sustained motor function. Thus, fatigue has become a limiting factor in limb reanimation projects that have attempted to use electrical stimulation.
The other type main function of the peripheral nervous system, sensory feedback, is responsible for pain, touch, appetite and a variety of other aspects. When problems with arise with sensory feedback mechanisms, the results are often drastic and sometimes even life threatening. For example, chronic pain is a serious health issue that affects many individuals, seriously degrading their quality of life and often having long-term psychological impact. Another issue addressable through sensory feedback relates to appetite suppression.
Aspects of the present invention relate to control and/or stimulation of peripheral nervous system using optical stimulus.